The high-pressure discharge lamp includes a light-emitting tube and a pair of electrodes that face each other in the light-emitting tube, a distance between free ends of the two electrodes is short, and the high-pressure discharge lamp is similar to a point light source. Thus, the high-pressure discharge lamp is used together with an optical system, and serves as the light source of an exposure device or a projector.
One example of such high-pressure discharge lamp is disclosed in Japanese Patent Application Laid-Open Publication No. Sho 60-79659 (Patent Literature Document 1).
FIG. 9 shows the high-pressure discharge lamp of this prior art. A light-emitting tube 10 of the high-pressure discharge lamp has a light-emitting portion 11, which is formed in a generally spherical shape and positioned in the center, and sealing portions 12 formed at the opposite ends of the light-emitting portion 11. In the light-emitting portion 11, a cathode 21 and an anode 31, both of which may be made from tungsten or the like, are arranged such that they face each other. A light-emitting substance such as mercury or xenon is sealedly present in a light-emitting space of the light-emitting portion 11.
Core wires 22 and 23 coupled to the cathode 21 and the anode 31 respectively, are sealed in the sealing portions 12 and 12 with a metal foil (not shown) or a structure (not shown) that is made from materials having slightly different coefficients of thermal expansion and connecting between the light-emitting tube and the core wire.
When the lamp is emitting light, an electric current flows between the two electrodes, and the temperature of the electrodes becomes extremely high due to the radiation from plasma and the resistance heating. In particular, the temperature of the anode becomes very high, namely, 2,000 degrees C. or more. Thus, it is necessary for the anode to be made from a metal having a small vapor pressure at a high temperature and to have a large thermal capacity in order to suppress the temperature increase of the anode. For example, as shown in FIG. 9B, the anode to be employed is entirely made from tungsten.
An electrode module 30 is constructed by making a core wire insertion hole 33 at the rear end of the electrode 31, inserting the core wire 32 into the insertion hole 33, and fixing it therein. Thus, the difference between the inner diameter of the core wire insertion hole of the electrode and the outer diameter of the core wire is designed to be extremely small in order to prevent the core wire from falling out.
As such, the insertion of the core wire may be difficult or the core wire may not be inserted to a desired depth due to the manufacturing tolerance of the core wire and the core wire insertion hole, particularly, the manufacturing tolerance of the core wire insertion hole, and due to the offset between the center position of the core wire and the center position of the core wire insertion hole during the core wire inserting process. Also, if a larger force is applied for the press-fitting during the inserting process, an excessive stress may act on the end of the core wire insertion hole and it would cause defects, such as chipping and cracking.